The present invention relates generally to fasteners. In particular, the present invention relates to a quick turn fastener for use with a molded or extruded tubular boss.
The fastener of the present invention is particularly useful for attaching to a plastic molded or extruded part. Existing fasteners for use with plastic molded materials are generally intended for attachment to flat panels, and require a flat panel or a through-hole in order to work. Examples include existing quarter-turn, “Christmas tree” and snap-fit style fasteners. The existing fasteners work reasonably well for their intended applications, but are not practical for attachment to molded parts where no flat panel or through-hole is available.
A tubular boss is a very desirable attachment feature for injection molded parts. Metal screws are commonly used to directly fasten to the tubular boss, forming threads on the inside. Thread forming metal screws can provide a connection with a high resistance to pull out, but this may not be required by the design and it may be more important to have a tool-free or quick assembly without the need for screw-driving equipment or tools. Metal screws also have the potential to corrode, which can be very undesirable. Non-threaded tubular bosses are simple to injection mold or extrude. They allow the use of blind holes for an air tight barrier, and they offer substantial diameter to length support for bending strength and alignment of attached parts. Unfortunately, no good method exists for attaching a plastic fastener directly to a simple tubular boss using prior art fasteners. As a result, metal screws continue to be used due to a lack of alternatives, which allows corrosion and adds costs, complexity and assembly time.
Thread forming using plastic screws in place of metal has been attempted, since thread forming is possible if the male thread is made from a material that is higher strength than the tubular boss. One example is using a male screw made of 30% glass-filled nylon to thread form into a much softer polypropylene tubular boss. However, preventing stripped threads becomes more difficult due to generated heat that can soften the threads. There is also increased risk of vibration loosening, since maintaining screw clamp load is more problematic with plastic material used for both the screw thread and tubular boss. A fully threaded tubular boss does not do much to avoid these potential issues and ‘unscrewing molds’ add considerable cost and complexity to mold tool construction. Assembly time is still slow and requires tools, compared to snap fit and quarter-turn style fasteners used in flat panels. Cross threading, axial alignment to the tubular boss, and achieving a specific rotational position after assembly are all difficult to control.
A complete injection moldable fastening system is desirable, in which the fastener is incorporated directly into the injection molded product desired to be fastened, to speed assembly and eliminate screws. A few examples are wire guides, stand-offs and various stud designs but many other possibilities exist.
Accordingly, a need exists for an improved quick turn fastener and fastening method that solves these and other deficiencies in the prior art. Of course, the present invention is not limited to plastics, may be used in softer metals and a multitude of situations where similar performance capabilities are required.